Magnetospheres of hot Jupiters: hydrodynamic models & ultraviolet absorption

TL;DR

This study uses hydrodynamic simulations to explore stellar wind interactions with hot Jupiter magnetospheres, predicting UV absorption features and assessing their consistency with observations, while evaluating cooling processes and applicability to other exoplanets.

Contribution

The paper introduces detailed hydrodynamic models of hot Jupiter magnetospheres, analyzing UV absorption signatures and the role of radiative cooling, which advances understanding of star-planet magnetic interactions.

Findings

Magnetospheres create large cavities of 6-9 planetary radii.

Weak bow shocks produce broad UV absorption features leading optical transits.

Radiative cooling is inefficient, unlikely to cause observed UV absorption.

Abstract

We present hydrodynamic simulations of stellar wind-magnetosphere interactions in hot Jupiters such as WASP-12b. For fiducial stellar wind rates we find that a planetary magnetic field of a few G produces a large magnetospheric cavity, which is typically 6-9 planetary radii in size. A bow shock invariably forms ahead of the magnetosphere, but the pre-shock gas is only mildly supersonic (with typical Mach numbers of $\simeq$1.6-1.8) so the shock is weak. This results in a characteristic signature in the ultraviolet light curve: a broad absorption feature that leads the optical transit by 10-20% in orbital phase. The shapes of our synthetic light-curves are consistent with existing observations of WASP-12b, but the required near-UV optical depth ($\tau \sim 0.1$) can only be achieved if the shocked gas cools rapidly. We further show that radiative cooling is inefficient, so we deem it unlikely that a magnetospheric bow shock is responsible for the observed near-UV absorption. Finally, we apply our model to two other well-studied hot Jupiters (WASP-18b and HD209458b), and suggest that UV observations of more massive short-period planets (such as WASP-18b) will provide a straightforward test to distinguish between different models of circumplanetary absorption.